1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, etc. or a multi-functional image forming apparatus, and more particularly to a cleaning device that removes residual toner on an image bearer of the image forming apparatus.
2. Discussion of the Background
Recently, demands for enhancing a quality of an image formed by an electrophotographic image forming apparatus has been increased. In order to enhance an image quality, it has been found that reducing a particle diameter of toner, uniforming a toner particle diameter, and enhancing a degree of a spherical shape of the toner are important. In an image forming apparatus for forming a full-color image, it has been found that the glossiness of an image affects an image quality.
Toner prepared by a polymerization method is known as the toner that satisfies the above-described demands. With the use of the toner prepared by the polymerization method, the image quality can be effectively enhanced.
However, when the toner prepared by the polymerization method is used in a conventional image forming apparatus, and transfer residual toner remaining on an image bearer after transferring of the toner image is removed by a cleaning member, a removal efficiency of the toner, that is, a cleaning efficiency may be deteriorated. For example, a cleaning blade whose edge is in press-contact with the surface of the image bearer is used as the cleaning member. When the transfer residual toner is removed from the image bearer by the cleaning blade, a space between the surface of the image bearer and the edge of the cleaning blade is in a most densely filled state with the transfer residual toner. A first layer of the toner having a strong adhesion force to the image bearer and a second layer of the toner slip against each other, and the first layer of toner adhering to the image bear surface sometimes passes the edge of the cleaning blade. As a result, the first layer of toner remains on the image bearer, thereby causing a cleaning defect.
In order to efficiently remove residual toner from an image bearer, Japanese Laid-open Patent Publication No. 5-265360 describes a cleaning method and device in which residual toner on a surface of an image bearer is removed by a cleaning blade press-contacted against the surface of the image bearer. The cleaning blade includes a conductive member. The residual toner on the surface of the image bearer is removed by applying alternating current (AC) bias voltage, and direct current (DC) bias voltage which has the same polarity as an electrostatic charge of toner at the time of developing, to the cleaning blade.
In a case of using a cleaning blade, when toner containing polyester-based toner binder having a small particle diameter, spherical shape, and melt viscosity lower than that of conventional toner is used in an image forming apparatus, the wax in the toner bleeds out and adheres to an image bearer due to the pressing force of the cleaning blade against the toner on the image bearer. As a result, a so-called wax filming, in which a film of wax of toner adheres to a surface of an image bearer, typically occurs.
FIG. 29 is a graph showing a relationship between the coefficient of friction of an image bearer and an operation time of an image forming apparatus when a wax filming occurs and does not occur. As seen from FIG. 29, when the wax filming occurs, the coefficient of friction of the image bearer decreases. When the coefficient of friction of the image bearer decreases due to the wax filming, the adhesion force between the image bearer and the toner is reduced by the wax filming. As a result, the toner on the image bearer is easily transferred to a transfer material such as a transfer sheet before a nip part formed between the image bearer and the transfer material by electric discharge, thereby causing an image deterioration such as toner scattering.
Further, a spherical dry toner, which contains at least modified polyester having a urea bond as a toner binder, and contains a wax finely dispersed in the spherical dry toner, has a property similar to a property of the toner prepared by a polymerization method. When the spherical dry toner is used, a high-quality image may be formed. In addition, when a toner image transferred to a transfer material is fixed by a fixing device, the toner image may be fixed at a relatively low temperature. Further, a hot offset phenomenon in which the toner attaches to a fixing member may be typically avoided. Furthermore, when the spherical dry toner is used in an image forming apparatus for forming a full-color image, color reproducibility, transparency, and glossiness stability of the image may be enhanced. However, even with the use of the spherical dry toner, because the particle diameter of the spherical dry toner is small, a problem similar to the problem with the use of the toner prepared by a polymerization method typically occurs.
In recent years, in the image forming apparatus, a contact type charging member has been often used as a charging member for charging the image bearer. The charging member abuts on the image bearer surface, causes electric discharge between the charging member and the image bearer, and charges the image bearer. When the contact type charging member is used, an amount of ozone generated during the electric discharge can be advantageously reduced.
However, when the contact type charging member is used, and the cleaning blade removes the toner prepared by a polymerization method from an image bearer, the cleaning efficiency is deteriorated as described above. When the amount of the transfer residual toner passed through the charging member increases, the toner adheres to the charging member, and the charging member is stained by the toner. Therefore, a charging function of the charging member is deteriorated, and the useful lifetime of the charging member is reduced.
Particularly, when a press contact force of the edge of the cleaning blade onto the image bearer is locally decreased, the amount of the transfer residual toner passed through the corresponding portion increases, and the charging member is partially and extremely stained by the passed toner. When the charging member is partially stained in this manner, the charging efficiency is locally deteriorated with respect to the surface of the image bearer, and the quality of the toner image is partially deteriorated. Therefore, it is necessary to replace the charging member partially and remarkably stained by the toner with a new charging member. In the conventional image forming apparatus, the partial stain of the charging member occurs in a relatively early stage, which thereby causes the useful lifetime of the charging member to decrease.